1. Field of the Invention
This invention generally relates to a touch system and, more particularly, to an optical touch system and a positioning method therefor.
2. Description of the Related Art
Please referring to FIGS. 1a and 1b, FIG. 1a shows a schematic diagram of an optical touch system and FIG. 1b shows a schematic diagram of image windows acquired by the two image sensors included in the touch system shown in FIG. 1a. 
The touch system 9 includes a touch surface 90 and two image sensors 91 and 91′. The image sensors 91 and 91′ are configured to acquire image windows W91 and W91′ respectively looking across the touch surface 90. When a finger 81 is hovering above or touches the touch surface 90, the image windows W91 and W91′ acquired by the image sensors 91 and 91′ respectively include finger images I81 and I81′ of the finger 81. A processing unit 92 can calculate a two-dimensional coordinate of the finger 81 with respect to the touch surface 90 according to a one-dimensional position of the finger image I81 in the image window W91 and a one-dimensional position of the finger image I81′ in the image window W91′.
However, when a plurality of fingers are hovering above or touch the touch surface 90 simultaneously, one finger may block other finger or fingers with respect to a part of the image sensors. For example in FIG. 1a, when two fingers 81 and 82 are hovering above or touch the touch surface 90, the image sensor 91 acquires images of the fingers 81 and 82 following a route “a” and the image sensor 91′ acquires images of the fingers 81 and 82 following routes “b” and “c” respectively. To the image sensor 91, as the finger 81 blocks the finger 82, the image window W91 acquired by the image sensor 91 only includes a finger image I81+I82 (i.e. a combined image of the finger images I81 and I82). Therefore, the processing unit 92 is not able to correctly calculate two-dimensional coordinates of different fingers with respect to the touch surface 90 according to the finger images I81′, I82′ and I81+I82.
Accordingly, it is necessary to provide a positioning method for an optical touch system that can correctly position a plurality of pointers.